Wireless communication systems are widely deployed to provide various communication services and location services to mobile users. Users in the system rely on position determination functionality to provide location services including navigation, location-based services, and point-of-interest applications.
A number of techniques exist to determine position of an access terminal in a wireless communication system, including Global Positioning System (GPS) techniques, Assisted GPS (A-GPS), and cell-based positioning methods such as Cell of Origin (COO), Time of Arrival (TOA), and Angle of Arrival (AOA). These techniques have varying degrees of precision, which may not provide the accuracy needed by many of today's location-based services. For example, GPS positioning can be especially inaccurate in urban environments, where tall, densely packed buildings can restrict views of satellites and the reflective surfaces of buildings can cause multipath effects.
With advances in sensor technology, electronic magnetometers are becoming standard equipment in access terminals, performing the function of classic magnetic compasses in navigation. If magnetic devices are used to determine direction or position, it is important to apply two corrections to the magnetic readings to express measured angles with respect to True geographical North. The two corrections are magnetic declination and magnetic deviation. It is well known that the Earth's magnetic field is not aligned with True North, and this misalignment depends on the location. The difference between True North and Magnetic North is called magnetic declination or variation. Very accurate models of the Earth's magnetic field can be used to provide the sign and value of magnetic declination. Additionally, the change in magnetic declination with time is well known for every place. Thus, magnetic readings can be corrected for magnetic declination, for example, by accessing publicly available magnetic declination data.
Magnetic deviation, the second correction, is a local disturbance of the Earth's magnetic field caused by metal objects added to the environment by human activity. Most civil engineering objects and modern buildings contain large amounts of steel in their structures. Each piece of the structure is magnetized and changes the direction of the Earth's local magnetic field. For a given location, the difference between Magnetic North and North shown by a magnetic device is called magnetic deviation.
Magnetic deviation in modern urban environments can reach very high values and can change rapidly from place to place, for example, over the distance of meters. Modeling magnetic deviation would require knowing and taking into account magnetic contributions of all pieces of hard and soft steel within a radius of up to about 50 meters, making it a very difficult if not impossible task. Yet not knowing and correcting for magnetic deviation can render magnetic device readings practically useless for navigation and guiding purposes in some environments.